[RFC] git-bisect unsure

I'm in the middle of tracking down a bug, unfortunately one of the  
commits that git-bisect chose had an unrelated problem in it that  
broke my build. As a result I don't know if this version is good or  
bad as far as the bug I'm trying to track down is concerned. I would  
think of a number of situations where one bug could interfere with the  
testing of another.

My proposal is to add an "unsure" option to git-bisect.

I'm thinking that when you say "git bisect unsure" it would simply  
shift one commit earlier or later and refrain from ever checking out  
one marked unsure until there were only "unsure" ones left at which  
point it would add a disclaimer when you called bad or good and throw  
up it's hands in frustration if you were to say "unsure" again.

-Kate